The present invention addresses problems in several areas which are seemingly unrelated without having the benefit of the disclosure concerning the present invention. A first area of the disclosure is the general area of frequency tunable antennas. Frequency tunable antennas are known to exist but such antennas do not provide a narrow bandwidth of operation. Moreover such frequency tunable antennas do not provide for system selectivity.
In typical communication systems, many communications channels are present. Each channel has a bandwidth commensurate with a single line of communication, whether it be digital data, voice, or other exchange of information. For example, channels for low baud rate narrowband FM signals typically employ bandwidths of 6.25 kHz, 12.5 kHz, or 25 kHz. Television channels typically occupy channel bandwidths of over 6 MHz. The size of the channel is application specific. It is important to point out that the antenna used in these systems will almost always have a bandwidth that is wide enough for a large portion of, if not at all, available channels to be received without retuning the antenna. For example, a dipole antenna typically has a useful bandwidth of about 10%. Although an antenna engineer would consider this to be a narrowband antenna, a communications engineer may consider it to be a wideband antenna if it allows most or all of the available channels of a specific system to be received, as the antenna imparts little if any channel selectivity to the overall receiver system.
The present invention also relates to electromagnetic bandgap (EBG) Artificial Magnetic Conducting (AMC) surfaces. AMC surfaces are also referred to as perfect magnetic conductor (PMC) surfaces and as high-impedance surfaces. When designing an EBG AMC ground plane, there exist certain intrinsic tradeoffs related to the frequency response and size of the structure. For example, when using a single-layer Frequency Selective Surface (FSS) mounted above a substrate backed with a Perfect Electrical Conductor (PEC) ground plane, the bandwidth of the resulting structure is strongly dependent upon the substrate thickness and effective dielectric permittivity. By increasing the substrate thickness with respect to wavelength, bandwidth can be increased. Also, by decreasing the relative dielectric constant of the substrate, the bandwidth can be further improved. Hence, the conventional approach for designing a broadband AMC surface has been to use a relatively thick substrate with a permittivity as close as possible to that of free space.
Such a structure is relatively straightforward to design and construct for operating frequencies above 1 GHz. This is due to the fact that at higher frequencies, a thick substrate in terms of wavelength can still be physically thin. This allows for a reasonable bandwidth on the order of 5 to 20% to be achieved with a physically thin structure. However, designing such a structure can become quite challenging for low frequency applications, specifically below 1 GHz. This is mainly because the substrate dimensions needed to achieve reasonable bandwidths of at least 5% or more are much too thick for most practical purposes. It is for this reason that EBG AMC structures are generally disregarded for low frequency applications.
Thus, problems remain with the use of EBG AMC structures and particularly to the low frequency application of EBG AMC surfaces as well as with frequency tunable antennas generally. Therefore, it is a primary object, feature, or advantage of the present invention to improve upon the state of the art.
It is a further object, feature, or advantage of the present invention to enable creation of an antenna system possessing generally narrow bandwidths such that the antenna system will screen out adjacent signals thereby providing radio system selectivity.
Yet another object, feature, or advantage of the present invention is to add tunability to an EBG to give overall antenna system frequency agility.
A still further object, feature, or advantage of the present invention is to create an ultra-thin EBG AMC structure with a high-k substrate material that operates effectively well below 1 GHz.
A still further object, feature, or advantage of the present invention is to use an ultra-thin EBG AMC structure with a high-k substrate material that operates effectively well below 1 GHz as the basis for creating a low-profile tunable narrowband (i.e., channel selective) antenna system.
Yet another object of the present invention is that it provides for limiting the bandwidth of an antenna such that it allows only one channel or a select group of adjacent channels through the antenna at any one time such that the antenna can be said to be narrowband and frequency selective with the antenna system adding frequency selectivity to an overall receiver system.
One or more of these and/or other objects, features, or advantages of the present invention will be apparent from the specification and claims that follow. The present invention is in no way limited by the background of the invention provided herein.